As an exhaust structure of a mono-block type lead acid storage battery which comprises a plurality of cells, there roughly are an individual exhaust system with an exhaust plug provided in each cell and a collective exhaust system for once leading the gas exhausted from each cell into an exhaust chamber and then exhausting it into the outside. In view of the advantages, such as needing less number of parts compared with the individual exhaust system, allowing the volume in an upper space of an electrode plate to decrease, allowing the battery to be downsized without reducing its volume, and allowing the leakage amount of electrolyte to be suppressed, the collective exhaust system has been broadly employed for on-vehicle lead acid storage batteries.
Conventionally, with respect to a lead acid storage battery which employs a collective exhaust system, a type that prevents the electrolyte leakage in a form of mist or vapor into the outside of the battery and exhausts the gas so that the inner pressure of the battery does not increase has been introduced. For example, batteries have been introduced having the structures, wherein the gas including electrolyte mist and moisture vapor flows in a horizontal direction within an exhaust chamber, while in this period, the electrolyte mist and the moisture vapor are dew-condensed so as to be separated into the gas and the electrolyte, and then the gas is discharged to the outside, while the dew-condensed electrolyte is recirculated within a cell (see Patent literatures 1 and 2).    [Patent literature 1]: U.S. Pat. No. 4,486,516 A    [Patent literature 2]: Japanese Unexamined Patent Publication No. H08-22815 A
Other than the above, there has been introduced a collective exhaust structure, wherein a distance for the gas flow in the horizontal direction is extended as long as possible in order to enhance the separation efficiency between the gas and the electrolyte (see Patent literature 3). In addition, an exhaust structure has been introduced, wherein a porous filter is disposed in a gas discharging port extended from an exhaust chamber to the outside of the battery so as to prevent discharging of electrolyte mist and moisture vapor, while at the same time, a space is provided in the lower side of the exhaust chamber so as to prevent clogging caused by the filter wet with electrolyte (see Patent literature 4).    [Patent literature 3]: Japanese Unexamined Patent Publication No. 2001-84981 A    [Patent literature 4]: Japanese Unexamined Patent Publication No. H06-176748 A
These lead acid storage batteries of collective exhaust system are effective in view of suppressing the leakage of electrolyte since they basically dew-condense the electrolyte mist and moisture vapor leaked from each cell to recirculate them in each cell. However, when the lead acid storage battery of collective exhaust system is used, for example, in an engine room, and when comparing the cells in both ends which are easily affected by heat generated from the engine with other cells (hereinafter referred to as “intermediate cell”) not easily affected by heat generation, a fall of the liquid surface occurs in the cells in both ends more easily than in the intermediate cells.
In short, when there is variation in leakage amount of electrolyte by each cell or in dew-condensed amount of electrolyte mist and moisture vapor, variation occurs also in the height of the liquid surface between cells. In a lead acid storage battery, there has been a problem that, with variation in the height of the liquid surface between cells, the life in a cell of low liquid surface height becomes shorter, thereby deteriorating the life performance as a battery.